With many vehicles it is desirable to perform the driving at two or more axles. When transporting heavy loads, especially by means of articulated vehicles, it is difficult to arrange a mechanical power transfer to axles located in a part of the vehicle, remote from the part carrying the prime mover.
Hydraulic transfer systems have been proposed, as well as combined systems where the prime mover drives upon one axle by conventional, mechanical gearing, whereas the transfer of torque to an axle, or axles, remotely located with respect to the prime mover has occured by way of a hydraulic transmission.
The prime mover has then been an internal combustion engine, and the hydraulic transmission has included a variable volume pump, necessitating a special system, or means, to adjust the capacity of the pump. Such systems are expensive and will easily suffer from operating troubles. These systems further include some kind of power divider, usually of a mechanical type, which allots a certain portion of the total output from the internal combustion engine to the hydraulic system.